We do sometimes see multiple governments competing in the same "markets". We usually call them "warlords" though, and it hasn't really worked out all that well for the people and lands being competed for.
I don't think the "rest of the world" really has any stake in it at all. A few hundred people do, at most. These people may not be completely harmless, but they also lack the capacity to hurt people in any significant numbers. If you can be hurt by a micronation, perhaps you need to reevaluate what your government is doing. How badly do they have to fuck up to the point that a micronation becomes relevant in your life?
In a sense, but they're subject to a tragedy of the commons (that's almost the definition of a negative externality), so competition does not improve them.